Thousands of Filipinos have flocked to the funeral of a schoolboy who was shot dead by police and dumped next to a pigsty to protest President Rodrigo Duterte's brutal war on drugs.

The killing of 17-year-old Kian Delos Santos last week triggered rare protests against Duterte's controversial but popular campaign to eradicate drugs, with critics saying it highlighted rampant rights abuses by police enforcing the crackdown.

Police said the teenager was a drug courier who fired at them while resisting arrest, however CCTV footage emerged of two policemen dragging the unarmed boy away moments before he was killed.

Since Duterte's term began 14 months ago, police have reported killing 3,500 people in anti-drug operations, with thousands more murdered over drug-related crimes and in unexplained circumstances.

Filipino schoolboy Kian Delos Santos (pictured), 17, was shot dead by police last week. They claimed he was a drug courier who fired at them, but CCTV appears to show he was unarmed

Thousands of people flocked to the teenager's funeral to protest President Rodrigo Duterte's brutal war on drugs (his flower-decked hearse is pictured)

Around 3,000 people including Santos' classmates, neighbours, nuns, priests and human rights activists marched under cloudy skies to protest his killing

A funeral worker shows media the body of Santos, whose murder has dominated the media and sparked public outrage

Duterte and his drug war are backed by a large majority of Filipinos fed up with high crime and a slow-moving judicial system, according to national polls.

But Delos Santos's murder has dominated the media and sparked public outrage.

After his family held a wake for him at home, around 3,000 people including his classmates, neighbours, nuns, priests and human rights activists marched under cloudy skies to protest his killing, according to an AFP photographer at the scene.

'Kian is the name and face of the truth. We must not allow the truth to die with Kian's murder,' said Father Robert Reyes, one of several Catholic priests who celebrated a church mass for the boy on Saturday.

Duterte, who had controversially drawn parallels between his drug campaign to Hitler's extermination of Jews and vowed to protect police from prosecution, has promised to bring the boy's killers to justice.

Santos' mother Lorenza is pictured weeping as she looks at the body of her slain son

Duterte and his drug war are backed by a large majority of Filipinos fed up with high crime and a slow-moving judicial system, but Santos' murder has sparked public outrage

The slow-moving procession snaked through narrow streets as participants, many wearing black ribbons, carried posters that read 'Stop Killing the Poor', 'Justice for Kian', and 'Rehabilitation not Persecution'.

The cortege stopped briefly for prayers outside a police station where the three officers who had arrested the boy were deployed. They have since been suspended.

Following their claims of Delos Santos being involved in the drugs trade, police told a public enquiry on Thursday that they only read about his alleged narcotics activity on 'social media' after his death.

A woman, whose son was also shot in a drug-related killing, cries during the funeral march of Kian delos Santos

A police autopsy also concluded the boy was fatally shot in the head twice as he lay prone on the ground.

Amnesty International alleged in a report released in February that Philippine police shot dead defenceless people, fabricated evidence, paid assassins to murder drug addicts, and stole from those they killed or the victims' relatives.

It also said police were being paid by their superiors to kill drug suspects, and documented victims as young as eight years old.

A police autopsy also concluded the boy was fatally shot in the head twice as he lay prone on the ground